1. Field of the Invention
The present invention relates to a dispersion-type electroluminescent element obtained using a film with transparent conductive layer on which a transparent conductive layer mainly composed of conductive oxide particles and a binder is formed and a method for manufacturing the same and particularly to a dispersion-type electroluminescent element applied as a light-emitting element incorporated in a key input component of various devices such as a cellular phone and the like and a method for manufacturing the same.
2. Description of the Related Art
The dispersion-type electroluminescent element (hereinafter abbreviated as “dispersion-type EL element” in some cases) is a light-emitting element by alternating current driving and is used for a backlight and the like of liquid crystal display in a cellular phone, a remote controller and the like and an application to a light emitting element incorporated in a key input component (key pad) of various devices has been recently tried as a new usage.
Such a device includes, for example, a portable information terminal and the like such as a cellular phone, a remote controller, a PDA (Personal Digital Assistance), a laptop PC and the like, and the light emitting element is used with the purpose of facilitating a key input operation in a dark place such as during a night.
ds the light emitting element of the key input component (key pad), a light emitting diode (LED) has been applied, but since there are problems such that the LED is a point light source, its brightness on a key pad portion is non-uniform and its appearance is poor, white/blue luminescent colors are generally preferred but those colors take a high cost in the LED, power consumption is larger than the dispersion-type EL element and the like, a trend to apply the dispersion-type EL element instead of the LED has become remarkable.
As a method for manufacturing such a dispersion-type EL element, the following methods are widely employed in general. That is, it is a method of sequentially forming a phosphor layer, a dielectric layer, and a rear electrode layer by screen printing and the like on a plastic film (hereinafter abbreviated as “sputtered ITO film”) on which a transparent conductive layer of an indium tin oxide (hereinafter abbreviated as “ITO”) is formed, using a physical film forming method such as sputtering, ion-plating and the like.
Here, as a paste used for application (printing) formation of each layer of the phosphor layer, dielectric layer, and the rear electrode layer, phosphor particles, dielectric particles and conductive particles are dispersed, respectively, in a solvent containing a binder, and a commercial paste, for example, may be used.
The sputtered ITO film is formed so that a ITO single layer, which is an inorganic component, is formed on a transparent plastic film of polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN) and the like by the above physical film forming method to have a thickness of approximately 20 to 50 nm, and a low resistance of approximately a surface resistivity: 100 to 300Ω/□ (ohm per square) is obtained.
However, since the ITO layer is an inorganic thin film and extremely fragile, a micro crack (split) can occur in the film, and in order to prevent that, a plastic film to be a base material needs to be provide sufficient strength and rigidity and its thickness is set at least at 50 μm or more, or usually 75 μm or more.
A PET film is now widely used for the base film of the above sputtered ITO film, but if its thickness is less than 50 μm, flexibility of the film is too high and a crack can easily occur in the ITO layer during handling, which extremely damages conductivity of the film. Thus, a thin sputtered ITO film with the thickness of approximately 25 μm, for example, has not been in a practical use. In the case of a soft base film made of urethane and the like, even if its film thickness is 75 μm or more, it can easily have a crack when the sputtered ITO layer is formed and has not been in a practical use.
Characteristics required when the dispersion-type EL element is applied to the keypad include, as described in Patent Document 1, for example, the above-mentioned uniformity in brightness and low power consumption and particularly, excellence in click feeling when the key pad is operated are important.
In order not to impair the click feeling when the dispersion-type EL element is incorporated in the key pad, the flexibility of the dispersion-type EL element itself needs to be sufficiently improved, that is, the thickness of the element is made as thin as possible or a flexible base film needs to be used.
However, if the dispersion-type EL element is manufactured using the above-mentioned sputtered ITO film, it is necessary to have a thickness of at least 50 μm or more for the base film in order to prevent a crack in the ITO layer so as to improve rigidity of the film, and the flexible base film can not be used. Thus, there is a problem that the click feeling of the key operation is not sufficiently favorable, if the element is applied to the key pad.
As another problem different from the above, Patent Document 2, for example, points out breakage/failure of an LCD (liquid crystal) component and the like caused by static electricity generated at a key input of a cellular phone. Thus, a similar problem might also occur in a key input component of the dispersion-type EL element, and as a measure against it, there is a method in which a transparent conductive layer is formed on an outer surface of the dispersion-type EL element, for example so as to have the static electricity escape, but since the base film for the key pad has high flexibility as mentioned above, it can not be applied to the conventional sputtered ITO film. Also, it is not easy to form an inexpensive transparent conductive film satisfying durability (hitting durability), transparency, conductivity required for the key pad on the outer surface of the dispersion-type EL element.    Patent Document 1: Japanese patent Laid-Open No. 2001-273831. with    Patent Document 2: Japanese patent Laid-Open No. 2002-232537.